The present invention relates to an exposure method and apparatus and more particularly to a projection exposure method and apparatus used in the lithographic operation for semiconductor memory devices and liquid crystal devices having regular fine patterns.
In the manufacture of semiconductor memory devices and liquid crystal devices by photolithographic techniques, the method of transferring a mask pattern onto a substrate has been generally used. In this case, the illuminating light for exposure purposes, e.g., ultraviolet light is irradiated on the substrate having a photosensitive resist layer formed on its surface through a mask formed with a mask pattern and thus the mask pattern is photographically transferred onto the substrate.
The common type of the fine mask patterns for semiconductor memory devices, liquid crystal devices, etc., can be considered as regular grating patterns which are vertically or laterally arranged at equal intervals. In other words, in the mask pattern of this type the most dense pattern area is formed with a grating pattern composed of equally-spaced transparent and opaque lines which are alternately arranged in the X-direction and/or the Y-direction to realize the minimum possible line width which can be formed on the substrate and the other area is formed with a pattern of a comparatively low degree of fineness. Also, in any case any oblique pattern is exceptional.
Further, the ordinary photosensitive resist material has a non-linear light response characteristic so that the application of a light quantity greater than a certain level causes chemical changes to proceed rapidly, whereas practically the chemical changes do not progress when the quantity of light received is less than this level. As a result, there is a background that with the projected image of the mask pattern on the substrate, if the difference in light quantity between the light and dark portions is ensured satisfactorily, even if the contrast of the boundary between the light and dark portions is low more or less, the desired resist image as the mask pattern can be obtained.
With the recent tendency toward finer pattern structures for semiconductor memories and liquid crystal devices, projection exposure apparatus such as a stepper for transferring a mask pattern onto a substrate by reduction projection have been used frequently and a special ultraviolet light which is shorter in wavelength and narrow in wavelengh distribution range has also come into use as an exposure illuminating light. In this case, the reason for reducing the wavelength distribution range resides in eliminating any deterioration in the image quality of a projected image due to the chromatic aberrations of the projection optical system in the exposure apparatus and the reason for selecting the shorter wavelength is to enhance the contrast of the projected image. However, the actual situation is such that this attempt of reducing the wavelength of an illuminating light has reached the limit with respect to the requirements for finer mask patterns, e.g., the projection exposure of line width of the sub-micron order due to the non-existence of any suitable light source, the restrictions to lens materials and resist materials, etc.
In the case of such a finer mask pattern, the required value for the resolution (line width) of the pattern approaches the wavelength of the illuminating light so that the effect of the diffracted light produced by the transmission of the illuminating light through the mask pattern cannot be ignored and it is difficult to ensure a satisfactory light-and-dark contrast of the projected mask pattern image on the substrate, thereby particularly deteriorating the light-and-dark contrast of the line edges of the pattern.
In other words, while the diffracted beams of the 0-order, xc2x1first-orders, xc2x1second-orders and higher-orders produced at various points on the mask pattern by the illuminating light incident on the mask from above are respectively reconverged at the corresponding conjugate points on the substrate for imaging through the projection optical system, the diffracted beams of the xc2x1first-orders, xc2x1second-orders and higher-orders are further increased in diffraction angle as compared with the diffracted beam of the 0-order and are incident on the substrate at smaller angles for the finer mask pattern. This gives rise to a problem that the focus depth of the projected image is decreased greatly and a sufficient exposure energy is supplied only to a portion of the thickness of the resist layer.
As a measure for coping with such decrease in the focus depth, Japanese Laid-Open Patent Application No. 2-50417 (laid open on Feb. 20, 1990) discloses the method of arranging an aperture stop concentrically with the optical axis of each of an illumination optical system and a projection optical system to restrict the angles of incidence of an illuminating light on a mask and adjusting the opening diameters of the aperture stops in accordance with a mask pattern. This ensures the focus depth while maintaining the light-and-dark contrast of a projected image on a sample substrate. Even in the case of this known method, however, the diffraction angles of diffracted beams of the xc2x1first-orders and higher-orders are still large as compared with a 0-order diffracted beam reaching substantially vertically to the surface of a substrate and practically all of them come out of the field of view of a projection lens, thereby producing on the substrate only a projected mask pattern image composed by substantially only the 0-order beam component and having a weak contrast.
Also, while, in this case, there is the possibility of a part of the xc2x1first-order diffracted beams coming within the field of view of the projection lens and reaching the substrate, in contrast to the 0-order diffracted beam incident substantially vertically on the substrate, the part of the xc2x1first-order diffracted beams is incident on the substrate at a smaller angle and therefore it is pointed out that a satisfactory focus depth is still not obtainable.
On the other hand, U.S. Pat. No. 4,947,413 granted to T. E. Jewell et all discloses a lithography system in which an off-axis illumination light source is used and an interference of the 0-order diffracted beam and only one of the xc2x1first-order beams from a mask pattern is made possible by use of a spatial filter processing in the Fourier transform plane within a projection optical system, thereby forming a high-contrast projected pattern image on the substrate with a high degree of resolution. With this system, however, the illumination light source must be arranged in an off-axis position in which it is obliquely directed to the mask, and also due to the fact that the 0-order diffracted beam and only one of the xc2x1first-order diffracted beams are simply caused to interfere with each other, the dark-and-light contrast of the edges in the pattern image resulting from the interference is still unsatisfactory due to the unbalanced light quantity difference between the 0-order diffracted beam and the first-order diffracted beam.
It is an object of the present invention to provide a projection exposure method and apparatus so designed that a projected image having a sufficient light-and-dark contrast is produced with a large focus depth on a substrate from the fine mask pattern of the ordinary mask having no phase shifting means, and more particularly it is an object of the invention to positively utilize the fact that the illuminating light has a narrow wavelength distribution, that the mask pattern can be substantially considered to be a diffraction grating, that the resist material has a non-linear light responsive characteristic for the amount of light received and so on as mentioned previously so as to form a resist image of a finer mask pattern for the same wavelength of the illuminating light.
In accordance with a basic idea of the present invention, when using an exposure apparatus including an illumination optical system for illuminating a mask formed at least partially with a fine pattern with an illuminating light and a projection optical system for projecting an image of the illuminated fine pattern on a substrate so as to transfer the fine pattern of the mask on the substrate, the illuminating light is directed from at least two locations to fall on the mask with given angles of incidence in an obliquely opposing manner so that the 0-order diffracted beam and either one of xc2x1first-order diffracted beams produced from the fine pattern by each of the obliquely illuminating beams are respectively passed through optical paths which are substantially equidistant from the optical axis of the projection optical system at or in the vicinity of the Fourier transform plane within the projection optical system with respect to the fine pattern on the mask, thereby forming on the substrate a projected image of the fine pattern principally by either of the xc2x1first-order diffracted beams and the 0-order diffracted beam. In this case, the other undesired beams excluding either of the xc2x1first-order diffracted beams and the 0-order diffracted beam do not substantially reach the substrate. As optical means for this purpose, principally spatial filter means is arranged in the illumination optical system and/or the projection optical system. Also, the illumination optical system can be constructed so as to direct the illuminating light along its optical axis and the illumination optical system includes an optical element, e.g., condenser lens means arranged on this side of the mask such that the illuminating light falls at the given angles of incidence on the mask.
An exposure apparatus according to a preferred aspect of the present invention includes an illumination optical system for irradiating an illuminating light on a mask, a projection optical system for projecting an image of the fine pattern on the illuminated mask onto a substrate and spatial filter means arranged at or in the vicinity of the Fourier transform plane within the illumination optical system and/or the projection optical system with respect to the fine pattern on the mask, and the spatial filter means includes at least two window means which are each defined by an independent limited area having a comparatively higher light transmittance than the surrounding at a position apart from the optical axis of the illumination optical system and/or the projection optical system in which it is arranged. The Fourier transform plane at which the spatial filter means is arranged is placed for example in a position that is practically in the pupil plane of the illumination optical system, the conjugate plane to the aforesaid pupil plane or the pupil plane of the projection optical system, and the spatial filter means can be arranged at least in one of these positions.
In accordance with another preferred aspect of the present invention, the spatial filter means includes the two window means at substantially the symmetric positions with the optical axis of the illumination optical system and/or the projection optical system in which it is arranged.
In accordance with another preferred aspect of the present invention, the number of the window means in the spatial filter means is 2n (n is a natural number). Also, the window means is preferably arranged at each of a plurality of positions determined in accordance with the Fourier transform pattern of the fine pattern.
In accordance with another aspect of the present invention, the illumination optical system includes an optical integrator, e.g., fly-eye lenses and in this case the spatial filter means is arranged in a position near to the exit end of the optical integrator.
In accordance with the present invention, the portion of the spatial filter means excluding the window means is generally formed as a dark portion or a light shielding portion whose light transmittance is substantially 0% or so or alternatively it is formed as a light attenuating portion having a predetermined light transmittance which is lower than that of the window means.
In accordance with another aspect of the present invention, the spatial filter means is arranged within the illumination optical system and the positions of its window means are selected such that either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam due to each window means are respectively passed through positions which are practically apart by the equal distance from the optical axis of the projection optical system at or in the vicinity of the Fourier transform plane within the projection optical system with respect to the fine pattern on the mask.
In accordance with another preferred aspect of the present invention, the spatial filter means is arranged within the illumination optical system and the spatial filter means includes first and second window means forming a symmetrical pair with respect to the optical axis of the illumination optical system, with the positions of the first and second window means being so determined that the two diffracted beams, i.e., either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the fine pattern by the irradiation of the illuminating light beam reaching the mask through the first window means and another two diffracted beams, i.e., either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the fine pattern by the irradiation of the illuminating light beam reaching the mask through the second window means are alternatively passed through separate first and second optical paths which are apart by practically the equal distance from the optical axis of the projection optical system at or positions near to the Fourier transform plane within the projection optical system, that is, the two diffracted beams, i.e., either one of the xc2x1first-order diffracted beams due to the illuminating light from the first window means and the 0-order diffracted beam due to the illuminating light through the second window means are for example passed through the first optical path and either one of the xc2x1first-order diffracted beams due to the illuminating light through the second window means and the 0-order diffracted beam due to the illuminating light through the first window means are for example passed through the second optical path.
In accordance with another preferred aspect of the present invention, the exposure apparatus includes drive means for varying at least one of the angular positions of the window means about the optical axis and their distance apart from the optical axis in accordance with the fine pattern on the mask for adjusting or switching purposes. Where the spatial filter means comprises a light shielding plate or light attenuating plate including a plurality of window means, the drive means comprises a mechanism for replacing the light shielding plate or the light attenuating plate with one having window means at different positions, whereas if the spatial filter means comprises an electrooptic element which is capable of making transparent or opaque the limited areas at arbitrary positions, such as, a liquid crystal device or an electro chromic device, the drive means comprises electric circuit means for driving the electrooptic element for the purpose of making the limited areas transparent or opaque.
The conventional projection exposure apparatus uses indiscriminately an illuminating light which falls at various angles of incidence on a mask from above so that the respective diffracted beams of the 0-order, xc2x1first-orders, xc2x1second-orders, and higher-orders produced from the mask pattern are directed in practically disordered directions and the positions at which these diffracted beams are imaged through the projection optical system on a substrate are different from one another. On the other hand, the projection exposure apparatus of the present invention selectively uses the illuminating light which is incident on a mask pattern with specified directions and angles from the given positions within a plane intersecting the optical axis at right angles so that either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the mask pattern by each illuminating beam are mainly directed onto the substrate and chiefly participate in the formation of a projected image of the fine pattern on the substrate. In other words, in accordance with the present invention the spatial filter means corresponding to the mask pattern is used for this purpose so that only optimum one of the xc2x1first-order diffracted beams and the 0-order diffracted beam by each illuminating beam are mainly selected from the illuminating light by the spatial filter means and are directed onto the substrate, thereby forming on the substrate a projected pattern image which is higher in the light-and-dark contrast of the edges of the fine pattern than previously and which is large in focus depth.
In this connection, there are the following two methods for the application of the spatial filter means according to the present invention. More specifically, the first method is such that the illuminating light is intercepted or attenuated at a portion of its beam cross-section on this side of the mask so as to select, as the principal illuminating light, the illuminating light obliquely incident with the specified direction and angle from each of the given positions within the plane intersecting the optical axis at right angles, and for this purpose the spatial filter means is arranged at the Fourier transform plane within the illumination optical system or a position near thereto. The second method is such that of the various diffracted beam components produced from the mask pattern illuminated by the illuminating light of various angles of incidence, the two component beams or either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the mask pattern by each of the illuminating beams incident obliquely with the given directions and angles from the given positions within the plane intersecting the optical axis at right angles are selected within the projection optical system, and for this purpose the spatial filter means is arranged at the Fourier transform plane within the projection optical system or a position near thereto. These first and second methods may be used in combination and in any way the spatial filter means serves the role of limiting the light beams participating in the formation of a projected pattern image on the substrate to either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the mask pattern by each of the illuminating beams which are incident obliquely with the specified inclination angles and preventing the other undesired beams from reaching the substrate.
Where the spatial filter means is arranged at the Fourier transform plane within the illumination optical system or a position near thereto, the illuminating light having a given wavelength is projected onto the mask pattern in the form of a diffraction grating with the given angles of incidence from the given eccentric positions in the given angular directions about the optical axis so that theoretically a series of spots due to the Fourier expanded 0-order, first-orders, second-orders and higher-orders diffracted beams are formed at the Fourier transform plane of the projection optical system or positions near thereto. In the conventional projection exposure apparatus, however, the second-orders and higher-orders diffracted beams are eclipsed by the lens tube of the projection optical system.
The spatial filter means arranged at the Fourier transform plane within the illumination optical system or a position near thereto is also designed so that the illuminating light falling substantially vertically on the mask is intercepted or attenuated and that the illuminating light to be incident on the mask at the given angles of inclination from the given eccentric positions in the given angular directions about the optical axis is selectively passed with a high light transmittance. In this case, if the diffracted beams of the second-orders and higher-orders are not desired, another spatial filter means is further provided at the Fourier transform plane within the projection optical system or in the vicinity thereof to block or attenuate these beams. As a result, a high-contrast projected pattern image is formed on the substrate by the 0-order diffracted beam and the first-order diffracted beams produced from the mask pattern by the illuminating light at the preferred angles of incidence.
Then, with the mask patterns for semiconductor memory devices and liquid crystal devices, there are many cases where the portion of the mask pattern requiring a high-resolution transfer has a pattern composed of a grating pattern in which basically equispaced transparent and opaque lines are regularly arranged alternately and this can be generally considered to be a repetition pattern of rectangular waveforms at the duty ratio of 0.5. Where the spatial filter means is arranged at the Fourier transform plane within the illumination optical system or a position near thereto, due to the diffracted beams produced from the grating pattern, a series of spots of the diffracted beams of the 0-order, xc2x1first-orders, xc2x1second-orders and higher-orders are formed at the Fourier transform plane of the projection optical system so as to be distributed in the direction of traversing the lines of the grating (the direction in which the lines are arranged). At this time, in the like manner known as the ordinary Fourier expansion of a rectangular wave, the 0-order diffracted beam provides a reference level for the light quantity in the projected image on the substrate and the xc2x1first-order diffracted beams are the light quantity variation components of the sinusoidal waveform having the same period as the grating, so that when these diffracted beam components are condensed on the substrate, the interference of these diffracted beams produces on the substrate an imaged pattern having a sufficient light quantity for the sensitization of the resist layer and a high light-and-dark contrast.
Also, in this case the ordinary mask pattern for semiconductor memory devices and liquid crystal devices can be considered to be a combination of a plurality of gratings which are respectively arranged vertically or transversely on the mask so that if spatial filter means is prepared so as to ensure illuminating light beams having the optimum eccentric positions in the angular directions about the optical axis and the optimum angles of incidence for each grating, the resulting Fourier pattern formed at the Fourier transform plane of the projection optical system forms a spot group arranged in the angular directions corresponding to the line arranging directions of the gratings and having the spacings corresponding to the wavelength of the illuminating light and the line pitches of the gratings. The light intensity of each spot is dependent on the number of pitches of the gratings and the orders of the diffracted beams.
As will be seen from this fact, the same effect can be obtained by arranging within the projection optical system spatial filter means formed with window means only at the positions corresponding to the required spot positions so as to select the diffracted beams directed to the substrate. In this case, the spatial filter means arranged at the Fourier transform plane or a position near thereto includes the window means at the spot positions of the useful diffracted beams in the Fourier transform plane so that the useful diffracted beams are selectively passed while blocking the undesired diffracted beams which cause deterioration of the contrast at the substrate surface.
Thus, the number and positions of the windows in the spatial filter means inherently differ depending on the mask pattern so that when the mask is changed, the spatial filter means is also changed in company therewith as a matter of course and moreover it must be exactly adjusted in position relative to the mask.
Next, a description will be made of the reason why the focus depth is increased by projecting the illuminating light beams of the given angles of incidence onto the mask pattern from the given eccentric positions in the given angular directions about the optical axis and forming an imaged pattern on the substrate by means of either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the mask pattern by each of the illuminating light beams.
Generally, where the substrate is in registration with the focal position of the projection optical system, the diffracted beams of the respective orders which emerge from one point on the mask and reach one point on the substrate are all equal in optical path length irrespective of the portions of the projection optical system through which they pass so that even in cases where the 0-order diffracted beam passes through practically the center of the pupil plane of the projection optical system, the 0-order diffracted beam and the diffracted beams of the other orders are equal in optical path length and, with the optical path length of the light beam passing through practically the center of the Fourier transform plane being taken as a reference, the difference between the optical path length of the light beam passing through any arbitrary position of the Fourier transform plane and the reference optical path length or the front wave aberration is zero. Where the substrate is in a defocus position which is not in registration with the focal position of the projection optical system, however, the optical path length of the diffracted beam having any of the first and higher orders and passing any closer-to-outer-periphery portion of the Fourier transform plane within the projection optical system to fall obliquely on the substrate is decreased as compared with the 0-order diffracted beam passing through or near the center of the Fourier transform plane when the substrate is positioned before the focal point and the amount of defocus is negative, whereas it is increased when the substrate is positioned in the rear of the focal point and the amount of defocus is positive; this difference in optical path length has a value corresponding to the difference in angle of incidence on the substrate between the diffracted beams of the respective orders and this is referred to as the front wave aberration due to the defocus. In other words, due to the presence of such defocus, each of the diffracted beams of the first and higher orders causes a front wave aberration with respect to the 0-order diffracted beam and the imaged pattern in either the front or the rear of the focal point is blurred. This front wave aberration xcex94W is given by the following equation
xcex94W=1/2 xc3x97(NA)2xc2x7xcex94f
where
xcex94f=the amount of defocus
NA=the value of the distance from the center in the Fourier transform plane given in terms of the numerical aperture.
As a result, in relation to the 0-order diffracted beam (xcex94W=0) passing through practically the center of the Fourier transform plane, the first-order diffracted beam passing through the position of a radius r1 near the outer periphery of the Fourier transform plane has the following front wave aberration
xcex94W=1/2xc3x97r12xc3x97xcex94f
and the presence of this front wave aberration is the cause of deterioration of the resolution before and behind the focal position and reduction in the focus depth in the conventional techniques.
On the other hand, in the exposure apparatus of the present invention the spatial filter means is arranged such that either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the mask pattern by each of the illuminating light beams of the given angles of incidence are respectively passed through the eccentric positions (having the same eccentric radius r2) of substantially the central symmetry in the Fourier transform plane within the projection optical system. As a result, in the case of the exposure apparatus of the present invention the front wave aberrations caused by the 0-order diffracted beam and the first-order diffracted beams before and behind the focal point of the projection optical system are both given as follows
xcex94W=1/2xc3x97r22xcex94f
and they are equal to each other. Thus, there is no deterioration (blur) of the image quality caused by the front wave aberrations due to the defocus, that is, the correspondingly increased focus depth is obtained.
On the other hand, where the spatial filter means is arranged within the illumination optical system, a pair of the illuminating light beams passed through the pair of the window means symmetric with the optical axis take the form of light beams which are incident on the mask surface obliquely and symmetrically on both sides of the normal so that each one of the xc2x1first-order diffracted beams produced from the grating pattern on the mask by these light beams passes a position which is symmetric with the 0-order diffracted beam with respect to the optical axis of the projection optical system and falls on the substrate at an angle of incidence as large as that of the 0-order diffracted beam. As a result, the substantial numerical aperture of the projection optical system participating in the imaging is reduced thereby ensuring a greater focus depth.
Thus, in accordance with the present invention, by virtue of the fact that the spatial filter means having the windows of the paired structure on both sides of the optical axis is used such that, of the diffracted beams produced from the fine pattern on the mask by the illuminating light beams of the preferred angles of incidence, the diffracted beams of the preferred orders, i.e., the 0-order diffracted beam and the first-order diffracted beams are selectively condensed at the same position on the substrate so that even in the case of such fine pattern which has never been resolved in the past, it is now possible to ensure a satisfactory light-and-dark contrast and a satisfactorily large focus depth for sensitizing the resist layer in the imaged pattern on the substrate without any change of the illuminating light and the projection optical system.
Where the spatial filter means is arranged within the illumination optical system, the spacing between the pair of windows in the spatial filter means is such that either one of the xc2x1first-order diffracted beams produced from the fine grating pattern of the mask by the illuminating light passing through one of the windows and the 0-order diffracted beam produced by the illuminating light passing through the other window are passed through substantially the same eccentric position in the Fourier transform plane of the projection optical system.
Where the spatial filter means is arranged within the projection optical system, the spacing between the pair of windows in the spatial filter means is determined in such a manner that either one of the xc2x1first-order diffracted beams and the 0-order diffracted beam produced from the fine grating pattern of the mask by each of the illuminating beams of the preferred angles of incidence are respectively passed through separate eccentric positions.
In accordance with the exposure apparatus of the present invention, a suitable adjusting mechanism is used so that the spatial filter means is rotated through a certain angle or parallelly moved within the plane of its arrangement so as to compensate for the shifts in the positions of the windows of the spatial filter means relative to the mask pattern. Also, the spacing between the pair of windows may be constructed so as to be adjustable to conform more satisfactorily with the Fourier pattern of the mask pattern. In this case, by constructing so that the positions of the windows in the spatial filter or the spacing between the windows can be varied by the adjusting mechanism, it is possible to obtain the optimal positional relation between the mask and the windows of the spatial filter and also it is possible to use the same spatial filter in common with other masks containing different patterns.
In accordance with another aspect of the present invention, a spatial filter incorporating an electrooptical element such as a liquid crystal device or an electro chromic device is employed so that the adjustment of the positions and size of its windows can be effected by means of electric signals. In this case, due to the fact that the limited areas at the arbitrary positions of the spatial filter composed of the electrooptical element can be freely adjusted to become transparent or opaque, it is possible to obtain the optical positional relation between the mask pattern and the windows of the spatial filter and in this case it is of course possible to use the same spatial filter in common with other masks containing different patterns.
In order to facilitate the understanding of the above and other features and advantages of the present invention, some preferred embodiments of the invention will be described hereunder with reference to the accompanying drawings.